The subject of the present disclosure relates generally to downhole wellbore systems used for pumping hydrocarbon products to surface. Such systems are often called artificial lift systems. The present systems typically use a progressive cavity (PC) pump to pump liquid hydrocarbon from underground formations in a cased wellbore up to surface. The stator portion of the PC pump is typically run down on a tubing string and the rotor portion of the PC pump is run into the stator on a rod string. Movement of the rotor within the stator creates a series of annular spaces through which fluid travels as the PC pump operates. Fluid is pumped from a lower inlet between the rotor and stator up through the annular spaces to surface.
In wells with high solids contend, the issue of build-up of solids blocking the inlet of the annular space, thereby preventing the pump from pumping.
To overcome this issue, and to wash out debris accumulating at the pump inlet end, hollow rotors have been used in the past, see for example U.S. Pat. No. 6,907,925 to Cote. This patent proposes a portion of the rotor being hollow with a central bore extending from a primary orifice near the pump primary inlet to a secondary orifice spaced away from the primary pump inlet. A means is provided for diverting a portion of the fluids being pumped into the secondary orifice and diverting down through the bore and out the primary orifice to thereby wash away accumulated solids from the primary pump inlet. However, to ensure that fluid enters at the secondary orifice and exits at the primary orifice, against the pressure of fluid in the pump, sufficient pressure drop or fluid resistance is required in the bore. In order to achieve sufficient pressure drop, the primary orifice is sized and or otherwise designed to restrict the primary orifice to thereby create the backpressure required to overcome the pressure of the PC pump itself and exit the primary orifice with sufficient pressure to wash away the debris. Such precise sizing of the primary orifice is rendered useless when fluid dynamics of the fluid being pump changes and the restriction becomes too small or too large to provide proper back pressure. Furthermore, the restrictive sizing of the orifice commonly leads to blockage of the orifice when larger particulate travels down the central bore. To avoid blockage of the bore itself with debris from the fluid being pumped, a screen or filter is required to be applied over the secondary orifice, and such screen leads to further potential blockages. Furthermore, it has been found that such a screen or filter itself cannot restrict all particulates.
Other devices, such as that taught in CA 2,510,240 teach an external circulation conduit with a venture style circulation nozzle for taking a side stream of fluid and re-direct it at the pump intake. However, external conduits have a tendency to become damaged during installation, or vibrate and shake as described on page 10, creating fatigue in the device, and need to be strapped or otherwise secured to pump to avoid it breaking off. Furthermore, while the circulation nozzle can be changed to change the restriction of the nozzle orifice, this still does not adapt to changes in fluid dynamics during a pumping operation. Pumping would need to be stopped and that pump string brought to surface for the nozzle to be changed each time fluid dynamics changed.
Further prior art systems, such as that shown in U.S. Pat. No. 7,290,608 to Wittrisch, teach a separate tube or line connected to a secondary fluid source and a pump, said separate line running through the rotor to pump a secondary fluid to the PC pump inlet. Such designs necessitate a completely separate system of secondary fluid storage, pumping and piping through the existing PC pump system. The secondary pump must be large enough to overcome the PC pump pumping pressure to ensure that the secondary fluid will in fact flow through the piping and exits the rotor under sufficient pressure to wash away debris. The system further adds additional surface equipment, piping and increased expense to the system.
A need therefore exists in the art for a device and method for keeping PC pump intakes free from debris and keeping agitation at the pump inlet steady to encourage circulation.